Depth sounders operate on the principle of transmitting a high frequency sound impulse down through the body of water the depth of which is required and receiving the echo signal bounced off the river or seabed. The time between start of the high frequency impulse and receipt of the echo is automatically measured and this is directly proportional to the water depth.
All known depth sounders incorporate a visual display of the depth and this requires that the operator shift his visual attention to the device whenever he wants to know the depth. If the vessel is in perilous water, e.g. entering an unknown harbour, particularly at night or during bad weather conditions, such distraction from the radar display or the surrounding area can prove fatal. This is particularly true in the case of night operations, with the inevitable loss of night vision caused by looking at an illuminated sounder display.
The problems are exacerbated when the depth sounder is operated by a commercial fisherman who must alternate between watching his course, working his gear, and looking at the display, so as to prevent losing his gear on a shallow bottom. However, every second he takes his eyes away from his work increases the likelihood of serious injury from hooks, nets, line winches, ect. Particularly in the case of trawlers, the fisherman is often required to tend his gear in the stern of the vessel and so it is virtually impossible for him to monitor the visual display.
Finally, conventional sounder displays are hard to see in bright sunshine, and in addition, the display must be housed within a waterproof enclosure to protect it.